This invention is an apparatus to control modulation pressure in a fourth gear clutch pack used on buses and other equipment. Modulation pressure in a fourth gear clutch pack increases to full pressure in fourth gear and decreases to minimum pressure in all lower gears because of how the flow of fluid out of the fourth gear clutch pack is regulated. In first, second, and third gears, the modulation pressure is at a low level which preferably ranges from 0.5 to 0.7 bars. In fourth gear, the modulation pressure is at fourth gear pressure which preferably ranges from 5 to 7 bars. Modulation pressure affects upshifting to fourth gear and downshifting from fourth gear. This pressure variation is necessary for the fourth gear clutch pack to operate and not burn out.
A review of the prior art does not suggest an apparatus to control the modulation pressure in a fourth gear clutch pack as described in the present invention.
One known prior art is a manual system, which operates off an internal cam, which in turn is moved by an external cable. The cam, when turned by an external cable, urges a spring-loaded valve into an end bore of the fourth gear valve body, closing off flow out of the fourth gear clutch pack and resulting in the pressure rising therein. This cable needs maintenance every 6,000 miles, which costs a minimum of $75,000 per 600 transmissions. This cable is difficult to adjuste and often sticks during the winter season because of freezing temperatures, ice, and snow.
Another known prior art is a system retrofitted by Renk Corporation. This system, like the present invention uses a solenoid to regulate the modulation pressure in a fourth gear clutch pack, but unlike the present invention, Renk uses the solenoid differently. In the retrofitted system, when the transmission downshifts from fourth gear, the solenoid is energized and moves an added valve member out of an end bore of a fourth gear valve body, thus permitting fluid to flow out of the fourth gear valve body and resulting in the modulation pressure dropping in the fourth gear clutch pack. Further, in the retrofitted system, if the solenoid fails or is not energized, a spring mounted to the valve member urges the valve member into the end bore, thus preventing fluid from discharging out of the fourth gear valve body and resulting in the modulation pressure rising. If this would happen when the transmission is in fourth gear, the vehicle could possibly jolt as the transmission is downshifted. This jolt could possible cause serious injury to people riding in the vehicle. They could be hurled forward through the front windshield if the driver of the vehicle stops quickly. This system also includes a diode and a relay system.
There is a definite need for an apparatus to control the modulation pressure in a fourth gear clutch pack which overcomes the problems associated with the known prior arts.